1. Field of the Invention
The present invention relates to a supercritical pressure once-through boiler in which boiler water transformed into steam in boiler furnace wall tubes is further heated in a superheater and is then to a main turbine.
2. Description of the Prior Art
Constant-pressure type supercritical pressure once-through units have been designed and constructed for use in base load operations, but to promote nuclear power generation and to accommodate for the difference in the demand for power during different seasons or during the date and at night, in the future, a load regulating capability including very frequent stopping and starting during the night will be required for such units, in view of the demand for efficient power systems.
At present, nearly all boilers employed for base load operations are constant-pressure operation boilers in which steam pressure for a load is constant. Whereas a turbine consists of a combination of nozzles and blades that can be deemed to be one closed flow passageway through which fluid flows, if the load on the system is reduced and the flow rate of steam becomes lower, then since the pressure of the steam at the inlet of the turbine is also lowered, in view of the operational relationship of the turbine and the boiler it is necessary to reduce the pressure at the inlet of the turbine. If the pressure of steam at the inlet of the turbine can be lowered, it would be reasonable to lower the pressure of the boiler, too (variable pressure operation), in view of economy.
A main steam system of a once-through boiler in the prior art is generally shown in FIG. 11.
In this figure, water coming from a condenser (not shown) is pressurized by a boiler feed water pump 1 and then heated in a high-pressure feed water heater 2 and an economizer 3. This heated feed water then passes through a boiler furnace wall tube 4, a boiler throttle valve 5, and superheaters 6 8, and thereby being further heated. During this period, the temperature of the heated feed water is regulated to a temperature that is necessary to operate a main turbine (high-pressure turbine) 9 by means of a temperature-reducer 7, and its pressure is regulated by the boiler throttle valve 5 (basically, only for a partial load).
In the above-mentioned principal steam system of a once-through boiler, the water flowing out of the boiler furnace wall tube 4 can be regulated to a pressure that is necessary to operate the main turbine 9 at a load lighter than a predetermined partial load. However, the pressure reduction caused by the choking operation of the boiler throttle valve 5 creates the following problems.
Firstly, since the boiler throttle valve 5 is used continuously under a severe operating condition of a large pressure difference, its life is short, it is necessary to replace the valve periodically, and hence cost associated with the valve is expensive in terms of maintenance.
Also, since the steam is subjected to equienthalpy change in the boiler throttle valve 5 and creates no usable work, a pressure reduction at this portion is in the form of plant loss.